Examples of a new type semiconductor laser include a semiconductor laser called quantum cascade laser, which is based on a transition between energy levels (intersubband transition) of carriers within a common energy band of a conduction band or a valence band.
An oscillation wavelength of the quantum cascade laser depends on an energy gap between two energy levels with respect to optical transition. Therefore, the oscillation wavelength can be selected from over a wide spectral range (from a mid-infrared region to a terahertz wave band), and it has been first substantiated that such a laser can be achieved with a structure for which an oscillation wavelength of 4.2 μm in the mid-infrared region is selected.
In recent years, along with demands for an electromagnetic wave resource of a terahertz wave band which is considered to be useful for bio-sensing or the like, there has been conducted a development of a long wavelength laser for which the oscillation wavelength is selected from a region having longer wavelength than the mid-infrared region.
The long wavelength laser has such a structure of a gain medium as to produce a gain in a frequency range thereof and has a structure called surface plasmon waveguide, in which optical confinement to the gain medium can be strictly performed. This is different from a conventional semiconductor laser in which an optical confinement by a dielectric cladding is performed.
Japanese Patent Application Laid-Open No. 2000-138420 discloses a method of using as a cladding a negative dielectric constant medium whose real part of the dielectric constant is negative. In this case, a waveguide mode guided by the cladding is an electromagnetic wave to which polarization oscillation of charge carriers called surface plasmon within the negative dielectric constant medium has contributed. There is no diffraction limit in the surface plasmon, and thus the majority of mode intensities can be confined to the gain medium.
The use of such a technique achieves a laser oscillation whose oscillation wavelength is 11.4 μm, which is shifted toward longer wavelengths.
Further, Appl. Phys. Lett., Vol. 83, 2124 (2003) discloses a method of arranging, as a cladding, negative dielectric constant media whose real part of the dielectric constant is negative on a top and a bottom of a gain medium. In this case, the waveguide mode guided by the cladding is also the surface plasmon. A gain medium having two negative dielectric constant media as a cladding enables confinement of much more mode intensities to the gain medium, compared with the case in Japanese Patent Application Laid-Open No. 2000-138420. With the use of such a technique, a laser oscillation whose oscillation wavelength is about 100 μm (3 THz), which is more shifted toward longer wavelengths, is achieved.
In the long wavelength laser with the structure called surface plasmon waveguide as described above, stabilization for obtaining a desired laser oscillation is examined. Japanese Patent Application Laid-Open No. 2001-291929 discloses a technique of stabilizing the oscillation wavelength as a distributed feed-back (DFB) structure in which different negative dielectric constant media are repeated in a propagation direction of the waveguide mode. On the other hand, in the terahertz wave band where the oscillation wavelength is more shifted toward longer wavelengths, a stabilization technology in electronic devices can be utilized. Jpn. J. Appl. Phys., Vol. 44, 7809 (2005) discloses a technique of stabilizing oscillation of 0.59 THz (511 μm) by inserting a resistor element into an outside portion of an antenna resonator which is an electronic device.
However, in a conventional long wavelength laser, a tunnel injection is used as a method for current injection into a gain medium. Therefore, there has been a problem that an operating point is not stabilized and therefore a laser oscillation is made unstable.
The operating point is not stabilized, because a negative differential resistance region appears in current-voltage characteristics (I-V characteristics) along with the tunnel injection. This is a phenomenon which is not found in a typified optical semiconductor laser. Accordingly, the technique disclosed in Japanese Patent Application Laid-Open No. 2001-291929 is not assumed to be a sufficient stabilization structure for the conventional long wavelength laser, and hence a stabilization structure in which the operating point can also be stabilized has been required. Further, the technique disclosed in Jpn. J. Appl. Phys., Vol. 44, 7809 (2005) is a stabilization technique in electronic devices, which causes a problem that a laser oscillation cannot be attained when the stabilization technique is used for laser as it is.